This invention relates to a thin film resistor for microelectronic devices. Particularly, it relates to a thin film resistor formed in an semiconductor integrated circuit (IC) device, having a wide range of resistivity, low temperature dependency, and high thermal stability and which is easy to fabricate.
As microelectronics penetrate rapidly into various fields of the industry, semiconductor devices such as silicon semiconductor ICs are playing an increasingly important role. Although silicon ICs are dominant in the products at present, ICs of compound semiconductors such as gallium-arsenide (GaAs) semiconductors having a higher mobility of carriers are becoming popular because of their higher switching speed and lower power consumption. For both semiconductor products described above, higher packaging density is also required to improve their performance and to minimize the size of electronic devices where the ICs are used.
For a microelectronic device, particularly for an IC, resistors are important elements. Polycrystalline silicon (polysilicon) reistors are widely used for silicon ICs, while bulk resistors are used for compound semiconductor ICs. Usually such resistors are formed in a substrate of an IC together with other elements such as capacitors and transistors, but there are several limitations to the characteristics and fabricating conditions resulting in various problems which need to be solved.
For example, polysilicon resistors are employed for high resistive loads for static memory cell circuits and inverter logic circuits. These resistors have a high resistance such as 10 M.OMEGA. to 10 G.OMEGA., and have very small dimensions such as 2 .mu.m wide, 3 .mu.m long and 0.3 .mu.m thick. Accordingly, the necessary resistivity is approximately 10.sup.5 to 10.sup.6 .OMEGA.cm. Such a high resistivity of polysilicon is attained by controlling the dopant concentration. However, variation of the resistivity of the polysilicon is very sensitive to the variation of dopant concentration. If the phosphorus dopant concentration varies from 1.times.10.sup.18 atm/cm.sup.3 to 4.times.10.sup.18 atm/cm.sup.3, for instance, the resistivity increases sharply by 10.sup.5 times. This sharp change of resistivity with respect to dopant concentration provides a serious difficulty in controlling the specified value of the resistance. Generally, it is considered that the resisitivity of the polysilicon is composed of the bulk resistivity of crystalline grains, the actual resistivity of the grain boundaries and the space charge potential barriers. The above matters have heretofore been considered in a report, for example, on page 682, Vol.ED.29, No.4, APL 1982, IEEE Transactions on Electron Devices by Nicky Chau Chun Lu and others. The resistivity also depends on the grain size and grain boundary density of the polysilicon. However, controlling of the grain size during the crystal growth is difficult and further grain growth in subsequent heating processes at above 900.degree. C. is unavoidable, resulting in a change of resistivity.
The polysilicon is easily influenced by the adjacent layers of the device. For instance, it is reported that in the case of a polysilicon layer covered with a silicon nitride (SI.sub.3 N.sub.4) layer formed by a plasma depositing method, the resistivity of the polysilicon layer decreased to 1/1000 times by heating at 450.degree. C. for 2 hours. It is considered that this is ascribed to the occurrence of fixed charges and the surface state of the interface between the polysilicon layer and the insulator (silicon nitride) layer.
Furthermore, if the surrounding area adjacent to a polysilicon region is heavily doped, the dopants contained in the region tend to diffuse into the polysilicon resistor. As a result, the marginal area of the polysilicon resistor along the boundary area is doped and looses its resistivity, leading to a reduction of the resistivity of the polysilicon resistor. Considering this reduction, the resistor should be designed to start with a higher resistivity, but this adversely affects the high density packing of the IC.
In addition, a precise patterning for forming resistors into a semiconductor substrate by a selective diffusion method is difficult. An ion implantation patterning can assure higher accuracy of the dimension of resistors, but this method can not be applied to a compound IC such as gallium-arsenide (GaAs) ICs, because the temperature of the subsequent heat treatment for the diffusion process is limited due to the high evaporation rate of the element material such as arsenic (As) and is not sufficient to activate dopants implanted, resulting in uncontrolled resistivity of resistors.
Particularly with compound semiconductor ICs, resistors for the ICs are formed in the substrate occupying the area on the substrate, thereby decreasing the packaging density.
In order to overcome the problem with polysilicon resistors as described above, heat resistive resistor materials have been developed. A typical one is "cermet" which is a mixture of oxide materials such as aluminum oxide (Al.sub.2 O.sub.3), beryllium oxide (BeO) and zirconium oxide (ZrO.sub.2) and metals such as iron (Fe), nickel (Ni), cobalt (Co), chrome (Cr), copper (Cu), etc. "Cermet" is a heat resistive resistor material which can stand heating at 1000.degree. C. However, metals contained in "cermet", tend to react with oxygen to convert to a metal oxide. This makes it difficult to control the resistance of "cermet" resistors. Furthermore, in a compound semiconductor, "cermet" is not useful because of the temperature limitation in its heat treatment as described above.
Another heat resistive material for film resistors or discrete resistors is tantalum nitride (TaN) which is moisture proof and stable during its lifetime. Recently, an improved resistor of tantalum nitride was proposed in Japanese patent application, No. TOKU-KAI-SHO58-14501 (published on Jan. 27, 1983) by K. TANAKA, wherein a tantalum-silicon-nitrogen alloy layer, containing silicon (Si) of 20 to 80 atomic percent, was formed in an atmosphere including nitrogen (N.sub.2) gas with partial pressure of 0.5.times.10.sup.-5 to 8.times.10.sup.-5 Torr. The material can be heated below 800.degree. C. This resistor is substantially a nitrided tantalum silicide resistor having a relatively low range of resistivity from 4.times.10.sup.-4 to 2.times.10.sup.-2 .OMEGA.cm. Accordingly, this is not suitable for silicon transistors.